This invention was made with government support. The government has certain rights in this invention.
1. Field of the Invention
The present invention relates generally to optical fibers and more particularly to a novel method for eliminating the temperature sensitivity of an optical fiber and also to a temperature insensitive optical fiber constructed in accordance with the novel method.
2. Description of Related Art
Presently known fiber optic sensors have a temperature sensitivity resulting from the temperature dependent change of optical path length in the optical fibers used to construct such sensors. The optical path length is a function of both the index of refraction of the fiber and its geometric length. The index of refraction is temperature dependent as is geometric length.
In the prior art, to compensate for temperature dependency of the fibers, active servos have been utilized. For example, one such active servo is a piezoelectric fiber stretcher which is placed in intimate contact with the optical fiber. An electrical signal is developed as a function of the ambient temperature, and this signal is applied to the piezoelectric stretcher. As is well known in the art, the piezoelectric mechanically deforms in response to an electrical signal. This mechanical deformation is coupled to the optical fiber in a predetermined relationship such that the optical path length of the optical fiber remains the same.
Such optical fiber sensors find many uses in fiber microphones, fiber hydrophones, fiber gyroscopes, fiber magnetic field sensors and fiber electric field sensors. Fiber interferomic sensors are becoming important in the industry because they are environmentally rugged and insensitive to electromagnetic interference.
At present, some instruments in automobiles are planned to be replaced with fiber sensors. Temperature gauges and fuel gauges for automobiles have the potential of being made relatively inexpensive using fiber optics. However, a significant limitation and disadvantage of the prior art, as discussed above, is that these devices are relatively expensive because of the use of the active servos used to stretch the fibers to achieve temperature insensitivity. For many applications, the added cost of piezoelectric stretchers prohibitively adds to the cost of the fiber sensors.
In phased array radars, the relative phase of the RF or microwave energy between the antenna elements can be controlled optically and ideally should be independent of temperature. Again, this temperature independence is not practically achieved within the prior art.
Fiber optic sensors can also be used for laser frequency stabilization which require the ratio of the changes in frequency to the desired frequency to be small, typically less than 10.sup.-14.
Present known fiber optic interferomic sensors measure temperature and pressure (acoustic or DC) as well as current, electric field strength and other parameters simultaneously. However, it is often difficult to separate the temperature produced effects from the other parameters one wants to measure.